1. Technical Field
The present disclosure generally relates to a display device, and more particularly, to an array of thin-film-transistor (TFT) of a display device.
2. Description of the Related Art
Flat panel displays (FPD) are employed in various electronic devices such as mobile phones, tablets, notebook computers as well as televisions and monitors. Examples of the FPD includes a liquid crystal display (LCD), a plasma display panel (PDP), an organic light emitting diode (OLED) display as well as an electrophoretic display (EPD). Pixels of the FPDs are arranged in a matrix form and controlled by an array of pixel circuits. Some of the driving circuits, which provides signals for controlling the array of pixel circuits, are implemented with thin-film-transistors (TFTs) on the same substrate as the array of pixel circuits. The substrate, where the pixel circuits and the driving circuits are formed on, is referred to as a TFT backplane.
The TFT backplane is an important part of a FPD as it functions as a series of switches to control the current flowing to each individual pixel. Until recently, there have been two primary TFT backplane technologies, one using TFTs with amorphous silicon (a-Si) active layer and the other using TFTs with polycrystalline silicon (poly-Si) active layer. Generally, fabricating a TFT backplane by using the amorphous silicon TFTs is cheaper and easier than making a TFT backplane with other types of TFTs. However, a-Si TFT has low carrier mobility, and thus making a high speed backplane for a display is difficult with a-Si TFTs.
To improve the mobility of the a-Si TFTs, a-Si can be subjected to a heat-treatment using a laser beam that anneals the Si layer to form polycrystalline silicon active layer. The material from this process is generally referred to as low-temperature poly-Si, or LTPS. The carrier mobility of LTPS TFTs is higher than the a-Si TFTs by as much as 100 times (>100 cm2/V·s). Even at a small profile, the LTPS TFT offers excellent carrier mobility, and thus it may be the ideal choice for fabricating a fast speed circuits in a limited space. Despite the aforementioned advantages, however, initial threshold voltages may vary among the LTPS TFTs in a backplane due to the grain boundary of the poly-crystalized silicon semiconductor layer.
Due to the polycrystalline nature of its active layer, however, LTPS TFTs tend to have larger variations in threshold voltage (Vth) among the TFTs in a backplane, which can lead to display non-uniformity referred to as “mura.” For this reason, a display drive circuit implemented with LTPS TFTs often requires an additional compensation circuit, which in turn, increases the manufacturing time and cost of the display.
A TFT employing an oxide material based semiconductor layer, such as an indium-gallium-zinc-oxide (IGZO) semiconductor layer (referred hereinafter as “the oxide TFT”), is different from the LTPS TFT in many respects. Oxide TFTs offer a higher carrier mobility than the a-Si TFTs at a lower manufacturing cost than the LTPS TFTs. Also, relatively low initial threshold voltage variations than the LTPS TFTs provide scalability to any glass size. Despite a lower mobility than the LTPS TFT, the oxide TFT is generally more advantageous than the LTPS TFT in terms of power efficiency. In addition, low leakage current of the oxide TFTs during the off state allows can be of a great advantage for designing a power efficient circuits. For instance, circuits can be designed to operate the pixels at a reduced frame rate when a high frame rate driving of the pixels is not needed.
However, stable high-yield production of oxide TFT based backplane requires optimization of the TFT design, dielectric and passivation materials, oxide film deposition uniformity, annealing conditions, and more. Solving one issue often means trading-off performance of another, and the degree of integration in a backplane of a display may become even lower than that of amorphous silicon or poly-silicon.
Accordingly, the maximum performance of a display cannot be obtained with a TFT backplane that is implemented with the same type of TFTs. Moreover, display itself can have various requirements, such as visual quality (e.g., luminance, uniformity), power efficiency, higher pixel density, reduction of bezel, and more. Meeting more than one of such requirements can be a difficult task with a TFT backplane implemented with a single type of TFTs.